Full Idea
The generality attaching to a nature - its relatedness to many particular instances - results from abstraction, so in this sense a generalized nature presupposes its instances, and does not, as Plato thought, precede them.
Gist of Idea
Particular instances come first, and (pace Plato) generalisations are abstracted from them
Source
Thomas Aquinas (Summa Theologicae [1265], Ch.5 Q85.2)
Book Reference
Aquinas,Thomas: 'Summa Theologicae (Concise)', ed/tr. McDermott,Timothy [Christian Classics 1991], p.136
A Reaction
This seems to be a quite explicit endorsement of abstractionism by Aquinas, despite all Geach's assertions to the contrary.